Under the “More” tab after the last line of text I beleive.
Your easiest bet being new is to add a post process volume from the list on the left side of the editor (search for post process) and with it selected in the world hierarchy check off “unbound” in the details panel (effects the entire world) or size it to your water area and then in your character do a branch if player is in volume post process volume then set vignette to 1.5 (true), false set it to 0.
Heres a example easiest way to fake being in water make change the hue to a dark blue too, you can play with the post process settings to get the “under water” look you want.
